Elongation cords, such as high-elongation cords, are used to reinforce the belt of a radial tire. This can be the radially outermost ply, also called a protection ply, or an intermediate layer. The protection ply is the ply which lies closest to the tread and thus to the surface. As a direct result of its position in a tire and as its name says, a protection ply fulfills a front line function in the protection of a tire: Every unevenness and every roughness on the roads are first felt and taken up by the protection ply.
The elongation cord can also be used In other parts of the belt as strips or covering the whole width of the belt. Very often the elongation cords are used under a nearly 0° angle with respect to the equatorial plane of the tire, so in a circumferential direction. In this way the elongation cord can stabilize the belt edges and also control the belt deformation e.g. In case of a wide belt (low aspect ratio tire). A typical example for that is the so-called super single drive axle tire, this is one single wide tire replacing the normal 2 tires on every side that transmit the motor power of a truck to the road.
Consequently, severe requirements are put on cords reinforcing these protection plies.
First of all, the elongation cords must have a high corrosion resistance, since moisture that is able to penetrate via cracks in the tread is most likely to arrive first at the protection ply. Full rubber penetration is a way to slow down the corrosion attack on steel cords. The elongation cords must have—as their name expresses it—a high elongation before they break.
Thirdly, the elongation cords must allow an easy manufacture of the tires when they are used under a nearly 0° angle with respect to the equatorial plane. For that reason a high elongation at low loads is required in order to follow the tire expansion when the green tire Is not present, the elongation cords risk to get into contact with steel cord of other plies.
Finally, the elongation cords must be cheap in manufacture.
An example of a high-elongation cord is the well-known 3×7×0.22 SS high-elongation (HE) steel cord with an elongation at fracture of about 7.5%.
Such a cord, however, is expensive to manufacture, due to its multi-strand construction and due to their small twisting steps, which are required to make the steel cord construction springy and obtain the final elongation.
The prior art has already provided alternative cords which provide the required elongation at fracture and which are less costly from a manufacturing point of view. However, while providing a cheaper alternative, prior art cords do not provide a good rubber penetration.